Could someone explain the correct way to measure scale-length, neck-width, string-spacing etc, so I can put the specs up for a guitar I'm selling. Many thanks.

For the scale there are 2 things to do. Measure from the nut to the 12th fret and double it. If it is a standard guitar it will be 2 x325 (650) or possible 2 x 320 (640). The check the complete length to the saddle. This will usually turn out to be 1-2mm longer. So it might be 651 on the treble and 652 on the bass. This however is a 650 scale not a 652. If it is an older guitar, you may have a longer scale and then also likely a wider fretboard but the principle is the same.

For the neck width, the easiest is to use a caliper. Typically you will have 52mm. The range there is likely 49mm to 54mm. People tend to giver this rounded to .5mm so you might see 51.5 or 52.5.

For the string spacing the caliper is also good. Some people might be interested in how much space there is from the edge of the fingerboard to the high e-string.